With the rapid development of optoelectronic technology in recent years, the electronic display devices become thinner and thinner while also provide high resolution, such as LCD (Liquid Crystal Display) or PDP (Plasma Display Panel). In general, TCP (Tape Carrier Package) and COF (Chip on Film) are most commonly used methods for packaging the driver IC chips of the thin display devices.
In the present processes of TCP/COF, the IC chips are mounted on the flexible substrate with leads (e.g., tape) and electrically connected to external electronic devices through the leads. Moreover, the chip packaging structures need to be tested and inspected for function checking and quality assurance after packaging. In order to test the electrical performance of the IC chips, the flexible substrate has a plurality of test pads connected to the leads configured thereon; that is to say, the testing instrument (e.g., probe card) can test the IC chips by probing these test pads.
To meet the trend of lightweight, thinness, small form factor and multifunction of the electronic devices, IC chips need to be shrunk in size but still have high integration density; accordingly, the number of test pads on the flexible substrate has to be increased greatly as well. However, due to the limited space available on the flexible substrate, test pads are usually arranged in a multi-row stepped arrangement for more test pads to be accommodated therewithin. Please refer to FIG. 1, which is a schematic diagram illustrating a TCP/COF chip packaging structure 1 according to the prior art. As shown in FIG. 1, the chip 16, the test pads 12, and the leads 14 connecting the chip 16 and the test pads 12 are configured on the flexible substrate 10 of the TCP/COF chip packaging structure 1. In prior art, the chip packaging structure usually has a large number of test pads on the output side (i.e., the test pads 12 disposed at the test pad area on the right side of FIG. 1); the test pads 12 hence are arranged in multi-row stepped manner within the limited space. In this arrangement, the leads 14 connecting to the test pads 12 also occupy a part of the test pad area; hence, the test pads 12 gradually become smaller in size while getting closer to the user area 100.
Furthermore, the cantilever probe card is commonly used for testing the TCP/COF chip packaging structures. In general, the probes of the cantilever probe card have an inclined distal end; therefore, needle slippage may occur when the probe card is forced downward to contact with the test pads, that is to say, the probe tips may slip and shift from the expected position. For large test pads such as the test pads 12 at the rightmost row (i.e., the outermost row) in the test pad area as shown in FIG. 1, probe contact can still remain within the test pad even though the needle slippage occurs, causing no effect on the electrical contact. However, for small-sized test pads such as the test pads 12 at the innermost row in the test pad area as shown in FIG. 1, the probe needles may slide out of the test pads when needle slippage occurs, leading to inaccurate testing results.